Array Questions at PhonePe: What to Expect

PhonePe’s technical interviews heavily emphasize array manipulation. With 70 out of 102 total coding questions tagged to arrays, mastering this data structure is non-negotiable. This focus stems from PhonePe’s core business in payments and financial data processing, where operations on sequences of transactions, user logs, or real-time data streams are fundamental. Efficient array handling translates directly to performant, scalable systems—exactly what interviewers assess.

What to Expect — Types of Problems

PhonePe’s array problems test a blend of algorithmic reasoning and practical implementation. Expect these categories:

1. Two-Pointer & Sliding Window: Dominant patterns for optimizing subarray or two-element problems. Common for tasks like finding pairs with a target sum, subarrays with a specific property (e.g., maximum sum, longest unique characters), or in-place manipulations.
2. Prefix Sum & Hashing: Frequently used for problems involving subarray sums (e.g., count of subarrays summing to k) or tracking cumulative states. Hashing (using dictionaries or sets) is crucial for achieving O(1) lookups to reduce time complexity.
3. Sorting & Searching: While not always explicit, many problems require sorted arrays as a preprocessing step to enable efficient two-pointer or binary search solutions. Variations include finding missing/duplicate numbers or merging intervals.
4. In-place Array Modification: Questions that demand O(1) extra space, such as moving zeros, removing duplicates from a sorted array, or applying rotations. These test your ability to manipulate indices carefully.
5. Simulation & Matrix Traversal: Multi-dimensional array problems that involve navigating grids in spiral order, rotating images, or applying BFS/DFS on matrices.

The difficulty often lies in combining these patterns within a single problem.

How to Prepare — Study Tips with One Code Example

Focus on pattern recognition, not memorization. Solve problems by first identifying the core technique. For example, if a problem asks for a contiguous subarray meeting a condition, think Sliding Window or Prefix Sum. Practice deriving time and space complexity for every solution.

A key pattern is the Two-Pointer technique for pair sum. Here’s how to implement finding two numbers in a sorted array that add to a target:

def two_sum_sorted(numbers, target):
    left, right = 0, len(numbers) - 1
    while left < right:
        current_sum = numbers[left] + numbers[right]
        if current_sum == target:
            return [left + 1, right + 1]  # 1-based indices
        elif current_sum < target:
            left += 1
        else:
            right -= 1
    return [-1, -1]

Recommended Practice Order

Build competence progressively:

1. Start with basic operations: traversal, insertion, deletion.
2. Master two-pointer fundamentals (pair sum, in-place modifications).
3. Move to sliding window for subarray problems.
4. Tackle prefix sum and hashing for subarray sum challenges.
5. Practice sorting-based and simulation problems.
6. Finally, solve hybrid problems that combine multiple patterns.

Consistency is key. Solve at least 2-3 array problems daily, focusing on clean, bug-free implementation under time constraints.

Practice Array at PhonePe